1. Field of the Invention
This invention relates to a cellular radio communication system; and it relates, in particular, to arrangements for detecting and responding to cochannel interference with respect to equipment in cells utilizing a common channel.
2. Description of the Prior Art
Cellular radio communication systems are being increasingly considered as arrangements which can allow substantially higher numbers of mobile radiotelephone subscribers to have access to a relatively limited number of radio communication channels. One form of such a cellular system is described in a book, High Capacity Mobile Telephone System Technical Report, Dec. 1971, prepared by Bell Laboratories, filed with the Federal Communications Commission in that month under Docket 18262, and copies of which can be purchased from Downtown Copy Center, 1730 K Street, N.W.; Washington, D.C. 20006. It has been proposed in such a cellular radio system to require each base station, sometimes called a cell site, which communicates on a set of channels in common with other base stations that are within a potential cochannel interference radius, to modulate onto its access, or call set-up, channel signals a supervisory tone which is unique to that base station among the group of possibly interfering base stations. Such a tone is transponded by each mobile unit receiving that channel. Any base station receiving the retransmitted tone employs it to determine whether or not the transponding mobile station has been captured by an interfering base station and whether or not the receiving base station has captured an interfering mobile station. However, supervisory tones have not been used for dealing with cochannel interference on voice channels, as distinguished from access channels; and specific circuits for accomplishing supervisory tone verification have not heretofore been available. Likewise, arrangements have not heretofore been available for utilizing such tones in a mobile unit for detecting and responding to cochannel interference.